1. Field of the Invention
The present invention relates to an image forming method and apparatus to form an image by using a scan optical system and, more particularly, to an image forming method and apparatus for converting input image data into dot information by using a predetermined threshold matrix, for controlling the scan optical system on the basis of dot information, and for forming an image.
2. Related Background Art
Hitherto, a method of expressing a half tone, a dither method, a density pattern method, or the like are known. According to those methods, an image having a screen angle can easily be formed due to a method of setting a threshold matrix. Further, in the reproduction of a color image which is formed by overlaying multicolor inks or multicolor toners, the above method is particularly useful to eliminate a moire fringe of the color and to obtain uniformity of an average color irrespective of a resist aberration or the like.
In addition to the above method, the applicant of the present invention has already proposed a method of improving a gradation while keeping a high resolution by a relatively simple construction of an apparatus. Such a method is called a PWM method and its principle and operation is shown in FIG. 12. In FIG. 12, an input digital image signal is converted into an analog image signal. By comparing the analog image signal with a periodic pattern signal such as a triangular wave signal, a pulse width modulated (PWM) binary signal is generated and the binary signal is used as a drive signal of a laser light source a laser printer.
On the other hand, there is a method intermediate the dither method and the density pattern method, wherein one pixel of a dither matrix is further finely divided and is converted into multi-values for each pixel by using a density pattern (hereinafter, referred to as a multi-value dither method).
The PWM method is applied to the multi-value conversion of each pixel, so that both a high resolution and a high gradation can be accomplished.
However, in the case where the well-known conventional processing method is applied to an image forming apparatus using the scan optical system (for example, a laser printer which forms an image by scanning a laser by using a polygon mirror), there is a drawback such that an image deterioration such as a pitch variation or the like occurs. In the conventional construction, therefore, particularly, in the case where an image having a screen angle was formed, a writing position is periodically deviated in each of the main-scan and sub-scan directions every scan line. As shown in FIG. 13, consequently, there is a drawback such that dots are connected (overlapped) and oblique lines are generated in, particularly, the highlight portion and the half-tone portion.
Such a drawback occurs due to the following reason. Namely, since the scan beam forming each dot has light amount distributions for the main scan and sub scan as shown in FIGS. 14A and 14B, when the beam is scanned to a position which is deviated from the position at which a dot should inherently be formed, the positions of the dots which should inherently be arranged at almost equal intervals as those of the peripheral pixels are deviated and, as shown in FIG. 15, an uneven interference occurs among the respective dots, so that such oblique lines are generated (consequently, as the resolution rises, such a drawback increases).
Such a problem is common to all image forming apparatuses, each of which forms an image having a screen angle by using the above scan optical system.